1. Field of the Invention
The present invention relates to a multi-layered mirror, a manufacturing method thereof, a stress control method thereof, and an exposure apparatus. More particularly, the present invention relates to a multi-layered mirror for X-rays or neutrons, such as X-ray apparatus including X-ray telescopes, X-ray lasers, and soft X-ray projection lithography.
2. Description of the Related Art
The complex index of refraction of a substance is expressed by the following equation:
n=1xe2x88x92xcex4xe2x88x92ixc2x7kxe2x80x83xe2x80x83(1)
The xcex4 and k parameters are each quite small relative to unity in the X-ray region, so reflective optical systems are used in the X-ray region. Within Equation (1), i2=xe2x88x921, and k of the imaginary term indicates the absorption of X-rays by the substance.
Lenses utilizing refraction can not be used in the X-ray region since the difference between the refractive index of substances in the X-ray region and the refractive index of a vacuum (=1) is extremely small. Furthermore, even when reflection is used, reflectivity is very low.
Since the refractive index is slightly below 1, a high reflectivity is displayed when X-rays strike a smooth surface at a small grazing incidence tilt angle since total reflection occurs below an angle (critical angle of total reflection) determined by the refractive index. This type of mirror is called a xe2x80x9cgrazing incidence mirror.xe2x80x9d
However, the X-ray reflection angle of a grazing incidence mirror is greatly restricted. Therefore, such a grazing incidence mirror is deficient because it is impossible to construct an optical system with the multiple elements required for correction of aberration.
Accordingly, by using multiple reflecting surfaces and by keeping the X-rays reflected from each reflecting surface in phase, a multi-layered mirror with a high reflectivity was developed. This multi-layered mirror uses a film using a first substance with a small refractive index (a first layer) and a film using a second substance with a large refractive index (a second layer). The two types of films are alternately applied upon a substrate
Whether the first substance layer or the second substance layer is adjacent to the substrate is not important. For example, a multi-layered mirror with molybdenum(Mo)/Silicon(Si) alternately multi-layered films has a reflectivity of 60% or greater for soft-X rays with 13 nm in wavelength and perpendicular incidence.
Because reflectivity is extremely small for nearly perpendicular incident angles that are even smaller than the totally reflecting limit angle xcex8c of a grazing incidence optical system, a reflective optical system used in the X-ray region uses a multi-layered film. Specifically, this type of multi-layered mirror is utilized in various fields of X-ray optics for applications such as X-ray telescopes, X-ray microscopes, X-ray reducing projection exposure devices, X-ray laser resonators, etc.
A multi-layered mirror used in such a reflective optical system is formed as laminated layers of two types of substances with high amplitude reflectivity at the interface. The thickness of each layer is determined based upon optical interference theory and is selected so that the waves reflected from each interface are in phase. Among the layered substances of such a multi-layered mirror, one substance has a small refractive index difference from vacuum (refractive index=1), and the other utilized substance has a large refractive index difference.
Since a multi-layered mirror can reflect X-rays perpendicularly, an optical system utilizing perpendicular reflection can have lower aberrations than a grazing incidence optical system utilizing total reflection. Furthermore, as indicated by Equation (2), a multi-layered mirror features wavelength selectivity since X-rays are strongly reflected only when the Bragg condition given below is satisfied:
2dxc2x7sin xcex8=mxc2x7xcex,xe2x80x83xe2x80x83(2)
where d is the multi-layered film periodic length, xcex8 is the incident tilt angle, xcex is the X-ray wavelength, and m is the order.
Previously known examples of a multi-layered film used for a multi-layered mirror include a W/C multi-layered film prepared by alternately laminating layers of W (tungsten) and C (carbon) and Mo/C multi-layered film prepared by alternately laminating layers of Mo (molybdenum) and C. Such multi-layered films are formed by thin film growth technology such as sputtering, vacuum deposition, CVD (Chemical Vapor Deposition), etc.
Among the multi-layered films used for such multi-layered mirrors, the long wavelength side of the L absorption edge (12.6 nm wavelength) of Si of a Mo/Si multi-layered film has high reflectivity, and a comparatively good multi-layered film can be produced that has 60% or greater reflectivity (perpendicular incidence) in the vicinity of 13 nm wavelength. Mirrors utilizing this Mo/Si multi-layered film are used for research in X-ray telescopes, X-ray microscopes, X-ray reducing projection exposure devices, X-ray laser resonators, etc. It is anticipated that such mirrors will be used for reduction copying lithographic technology utilizing soft X-rays, which is called xe2x80x9cEUVLxe2x80x9d (Extreme Ultraviolet Lithography).
While the sputtering method has produced high reflectivity Mo/Si multi-layered mirrors, thin layers formed by the sputtering method are known to generally have internal compressive stress (Sey-Shing Sun, Internal Stress in Ion Beam Sputtered Molybdenum Films, J. Vac. Sci. Technol. A4 (3), May/June, 1986). When such internal stress occurs within a Mo/Si multi-layered film, the internal stress causes deformation of a substrate of a multi-layered mirror. This deformation causes wave front aberrations in the optical system, resulting in degradation of optical characteristics.
Since the refractive indexes of substances are also extremely near unity for a neutron beam, multi-layered mirrors are used for neutron beams in the same marner as X-rays. Although perpendicular reflection is impossible due to the short wavelength (high energy) of a neutron beam, the critical angle of total reflection can be increased by formation of a multi-layered film upon a surface of a grazing incidence mirror.
Furthermore, in addition to X-ray and neutron applications, a multi-layered mirror of the present invention can be used for ultraviolet light, visible light, and infrared light.
The present invention is directed to a multi-layered mirror that substantially obviates one or more of the above problems due to the limitations and disadvantages of the related art, a manufacturing method thereof a stress control method thereof and an exposure apparatus.
An object of the present invention is to provide a multi-layered mirror with increased reflectivity.
Other objects of the present invention are to provide a multi-layered mirror with a multi-layered film with a lower internal stress and to provide a method of controlling internal stress.
Still another object of the present invention is to provide a method for manufacturing a multi-layered mirror with increased reflectivity and/or reduced internal stress.
A further object of the present invention is to provide an exposure apparatus that uses light with higher efficiency.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve these and other advantages in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes a multi-layered mirror that has a substrate, at least one first layer of a first substance, and at least one second layer of a second substance. The at least one first and the least one second layer are laminated alternately on the substrate. A difference between a refractive index of the first substance in a utilized wavelength region and that in a vacuum is large. A difference between a refractive index of the second substance in the utilized wavelength region and that in a vacuum is small. Thicknesses of the first and second layers are determined according to the utilized wavelength and the refractive indices of the first and second substances, respectively. For at least one pair comprising two adjacent first and second layers, at least one of the layer within the pair has a multi-layered structure formed by alternately laminating to the thickness of that layer, at least two substances whose real parts of complex indices of refraction in the utilized wavelength are approximately the same.
In another aspect of the invention, a multi-layered mirror includes a substrate, at least one first layer of a first substance, at least one second layer of a second substance, and at least one stress-relief layer formed in at least one of the at least one first layer or the at least one second layer. The at least one first layer and the at least one second layer are alternately laminated on the substrate. A difference between a refractive index of the first substance in the soft X-ray wavelength region and that in a vacuum is large, and a difference between a refractive index of the second substance in the soft X-ray wavelength region and that in a vacuum is small. When the at least one stress-relief layer is formed in the at least one first layer, it has approximately a same real part of a complex index of refraction in the soft X-ray wavelength region as the first substance. When the at least one stress-relief layer is formed in the at least one second layer, it has approximately a same real part of a complex index of refraction as the second substance.
In another aspect, the invention includes a multi-layered mirror having a substrate, at least one layer of silicon, at least one layer of molybdenum, and at least one layer of ruthenium formed within at least one of the molybdenum layers. The at least one layer of silicon and the at least one layer of molybdenum are alternately laminated upon the substrate.
Yet another aspect of the invention includes a multi-layered mirror having a substrate, at least one first layer of a first substance with a small refractive index, at least one second layer of a second substance with a large refractive index. The at least one first layer and the at least one second layer are alternately laminated on the substrate by sputtering and an interface between the at least one first layer and the at least one second layer are irradiated by an ion beam and smoothed.
In a further aspect, the invention includes a multi-layered mirror having a substrate, a first layer of a first substance formed on the substrate, a second layer of a second substance formed on the first layer, a third layer of the first substance formed on the second layer, and a fourth layer of the second substance formed on the third layer. It also has a fifth layer of a third substance formed within a layer made with the first or the second substance. A difference in a refractive index in a utilized wavelength region and a refractive index in a vacuum of one of the first and second substances is large. A difference in a refractive index in a utilized wavelength region and a refractive index in a vacuum of the other substance is small. Thicknesses of the first and third layers are determined based on the utilized wavelength and a refractive index of the first substance, and thicknesses of the second and fourth layers are determined based on the utilized wavelength and a refractive index of the second substance. The third substance has approximately a same real part of a complex index of refraction in the utilized wavelength region as that of the layer within which the fifth layer is formed.
Another aspect of the invention includes a multi-layered mirror having a substrate, a first layer of a first substance formed on the substrate by sputtering, a second layer of a second substance formed on the first layer by sputtering, a third layer of the first substance formed on the second layer by sputtering, and a fourth layer of the second substance formed on the third layer by sputtering. A refractive index of the first substance is different from that of the second substance.
In another aspect, a method for manufacturing a multi-layered mirror on a substrate is provided. At least one first layer of a first substance with a small refractive index and at least one second layer of a second substance with a large refractive index are alternately laminated upon the substrate by sputtering. At least one of the at least one first layer or the at least one second layer is irradiated using an ion beam prior to forming another layer on top of that layer.
Another aspect of the present invention is a method of manufacturing a multi-layered mirror on a substrate. At least one first layer of a first substance and at least one second layer of a second substance are alternately laminated upon the substrate, where a difference between a refractive index of the first substance in the soft X-ray wavelength region and that in a vacuum is large and a corresponding difference for the second substance is small. At least one stress-relief layer is formed within at least one of the at least one first layer or the at least one second layer using a substance that has approximately a same size of a real part of a complex index of refraction as the substance of the layer within which it is formed. Particle beam irradiation is applied after forming of at least one of the at least one first layer, the at least one second layer, or the at least one stress relief layer.
In a further aspect, the invention includes a method of controlling stress of a multi-layered film formed upon a substrate by first laminating a first layer of a first substance on the substrate, applying particle beam irradiation on the first layer, and then laminating a second layer of a second substance on the first layer. The first substance and the second substance are different.
Finally, in another aspect, the invention includes an exposure apparatus that has an X-ray source for generating X-rays, an illumination optical system capable of directing X-rays from the X-ray source to an X-ray mask, and a projection optical system capable of directing X-rays from the X-ray mask to a photo-sensitive substrate so that a pattern of the X-ray mask is copied to the photo-sensitive substrate. The illumination optical system and the projection optical system are equipped with multi-layered mirrors, at least one of which is a multi-layered mirror of the present invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only are not restrictive of the invention, as claimed.